Steel alloy



Patented Aug. 25 1936 PATENT OFFICE STEEL ALLOY.

William R. Fleming, Newport, Ky., assignor to The Andrews Steel Company, Newport, KyL, a corporation of Kentucky No Drawing. Application January 2,1935,

Serial No. 107

6 Claims.

This invention relates to steel alloys. It has to do particularly with alloy steels designed for use in certain parts of stills utilized in the distillation of oils. However, it is capable of application in other fields of service. i

In stills of the type indicated, the relatively high temperatures and pressures encountered give rise to a tendency towards deformation which is particularly acute in such parts as still tubes and fittings for same. Likewise, the high temperatures and pressures of the fluids being handled and of the materials of the still parts during operation accentuate the tendency towards corrosion and erosion. Furthermore, the tendency towards scaling, which is particularly pronounced at those points of the still which are subjected to directly applied flames cooperates with the corrosiveand erosiveaction to thin the walls of the parts of the still which are thus afiected. These various factors coact to increase the danger of explosion and, since the industry is trending towards increased temperatures and pressures to render distillation processes more efficient, such factors are becoming more and more important.

In the prior art, the industry first'attempted to meet the enumerated sources of danger by the use of carbon steels. However, as increased temperatures and pressures became more prevalent, the inability of carbon steels to resist high temperatures and pressures and to withstand corrosion, erosion and scaling led to a search for alloy steels more suitable to meet these conditions.

One of the earliest steps in this direction was the adoption of chromium-nickel steels, wherein approximately 18 per cent chromium and 8 per cent nickel entered into the steel composition.

These chromium-nickel steels were found to be,

quite efficient but shortly proved to be too expensive. The constantly changing methods in the industry accentuated this drawback, since the necessity for frequent replacements in order to keep abreast of'developments in the art tended to increase the cost involved in the use of such an expensive alloy.

More recently, steels with approximately 5 per cent chromium have been used rather extensively, both with or without the addition of approximately .50 per cent of tungsten or molybdenum. Such alloy steels have been comparatively satisfactoryto meet the conditions of service referred to but they are relatively expensive, particularly because of the frequent changes of method and apparatus.

In short, there is a need for a relatively cheap sures, such as those to which' plugs, headers,

valves, shells, plates and tubes of stills ar'e nor- 1o mally subjected under present day conditions, without undue deformation due to such temperatures and. pressures and without undue corrosion, erosion or scaling.

Another object of this invention is to provide 15 such an alloy steel which is constituted of such materials that it can be produced at a substantially lower cost than such steels as those which depend upon the addition of substantial amounts of chromium or other high cost materials to impart the characteristics necessary for meeting the indicated conditions of service.

Another object of this invention is to provide an alloy steel having high resistance to deformation and, at the same time, being readily 25 purpose of increasing'its resistance to stresses applied for-a long time at elevated temperature, that is, to increase its creep resistance. Likewise, similarly small percentages of molybdenum 40 have been added to other alloy steels, for example, those with from 1 to 6 per cent of chromium. Since molybdenum has ordinarily been classed as a carbide forming element, it has been c'ustomary in the production of creep-resisting alloys to retain substantial percentages of carbon in the alloy to insure the formation of molybdenum carbide particles or the like in dispersion throughout'the alloy.

I have departed from the prior art, in this respect, by avoiding the use of carbide forming elements other than molybdenpm Thus, as contradistinguished from the prior art, I hold thecarbon and manganese down as low as practicable in order to-avoid the presence of carbide partlcles. Thus, I am able to obtain an increased ingots.

stability of the steel and of its properties, since carbide particles, other than molybdenum carbide, tend to spheroidize, so that the creep-resistance tends to fall. Molybdenum carbide is relatively stable in this respect and it has been found that my molybdenum steels are resistant to change at high temperatures and pressures.

In the formation of my alloy steels, I preferably use substantial percentages of silicon. Thus, I am able to produce the silicon-molybdenum steel, wherein the combination of silicon and molybdenum increases the resisting properties of the steel with respect to corrosion, erosion and scaling, and also imparts valuable high temperature properties, such as resistance to deformation. In one form of my invention, I add to a very low-carbon steel of the ingot iron type, containing less than 0.05 per cent carbon, which is also low in manganese, containing less than 0.07 per cent manganese, silicon ranging in percentage from 1 to 2 per cent and molybdenum ranging in percentage fromd25 to .75 per cent. These ranges of percentages can be extended considerably while still retaining some or all of the advantages of the particular ranges given and without material increase in cost. Thisis particularly true with respect to the molybdenum content, which may range from a perceptible amount up to 1 percent.

To illustrate the deformation values of my silicon-molybdenum steel in comparison with chromium-molybdenum steel, certain creep tests were made.

The Si-Mo steel was cast in 18- by 22 inch billets, heated to 1950" F., and then forged to 2 -inch squares. The squares were dropforged in a closed die and allowed to cool in the air. The bars were then reheated to 1650 F. for four hours and air cooled.

The approximate analysis of this steel was as follows: C 0.04, Mn 0.15, S 0.025, P 0.01, Si 1.50 and Mo 0.66.

The chromium molybdenum 'steel contained the following constituents: C 0.14, Mn 0.40, S 0.015, P 0.015, Si 0.35,. Ni 0.25, Cr 4.60 and Mo 0.55.

The tests were made over a period of from 1000 to 1500 hours and, as is customary, there was computed from the results of these tests the percentage of deformation in 10,000 hours. The

' results of these comparative tests are shown by the following data:

Load lbs. per sq.

7 creep 111 10,000 hrs.

0 4-6 Cr .50 Mo It will be seen from this table that the chromium steel, when subjected to a temperature of 1000 F. under a load of 9200 lbs. per sq. in. de-

formed at a rate computed to produce a total The ingots were rolled to 4 by 4 inch e DJ 03 perature of 1000 F. showed only .37 per cent deformation. Also, it will be seen from the above table that my silicon-molybdenum steel when subjected to a load of. 3000 lbs. per sq. in. at the clearly shown by these tests, as indicated by the following table:

Yield Tensile Temp. Percent Percent Steel 01 test 552755 gg 'g elong. in reduction F. Zin. of area 4-6 Cr .50 Mo..- 1000 17100 41300 40.0 79.9 1.5 Si .65 Mo 1000 29300 57070 31. 2 72. 0

H C1 Mo.-- 1100 15150 32500 45. 5 .86. 5 1.5 Si .05 Mo 1100 27200 46370 33. 2 79. 2

4-6 Cr .50 M0 1200 12000 25100 66.8 87.2 1.5 Si .65 Mo 1200 19400 31600 44.6 82.6

1 Stress to produce permanent set of 0.2 percent.

It will be seen from the above that these short time tensile tests showed that my'silicon-molybdenum steel, in comparison with the chromiummolybdenum steel, possesses a marked superiority in yield strength and tensile strength while, at the same time, possessing adequate ductility.

It will be seen from the above that the preferred constituents of my silicon-molybdenum steel may vary considerably but it is desirable that the silicon be present in not less than 1 per cent and not more than 2 per cent by weight of the batch. Likewise, it is desirable that the molybdenum be present in percentages ranging from 0.25 to 1.00 per cent. It is also desirable that carbon in the'steel be no more than 0.05 per cent-manganese no more than 0.20 per cent, sulphur no more than 0.06 per cent and phosphorus no more than 0.06 per cent.

It should be noted that I have been able to obtain a high creep resistance by the use of molybdenum, practically in the absence of carbon and, therefore, without the formation of appreciable carbides, which tend to spheriodize and lower the creep resistance and which tend to enhance segregation and brittleness. Furthermore, the high creep resistance values which I have been able to obtain have been obtained without the use of chromium but, instead by the use of relatively low cost materials.

The silicon present in my steel serves to increase its resistance to corrosion and the substantial elimination of carbon furthers the action of the silicon in the elimination of the corrosion, since the higher the carbon content the greater the danger of corrosion.

The silicon present also serves to prevent erosion. Likewise, it inhibits the formation of iron oxide, so that scaling is decreased, while the scale actually produced is more adherent and, therefore, less harmful. a

It should also be noted that my material possesses high creep resistance and high tensile strength and that these properties are obtained without quenching, which is undesirable and expensive.

In short, it will be seen that I have provided a relatively cheap alloy steel which will replace chromium alloy steel in many parts of oil distillatton apparatus and render equal or superior service. It will likewise be apparent that the alloy steel which I have produced will be capable of use in other fields where high resistance to deformation, coupled with adequate tensile strength and resistance to impact are desirable.

Having thus described this invention, what I claim is:

1. A steel resistant to stress and to scalingat elevated temperatures containing carbon in .an amount less than 0.10 per cent, manganese in an amount less than 0.20 per cent, between 1 and 2 per cent silicon and between 0.25 and 1.00 per cent molybdenum, with the balance substantially all iron.

2. A steel resistant to stress and to scaling at elevated temperatures containing carbon in an amount less than 0.20 per cent, manganese in an amount less than 0.20 per cent, between 1 and 2- per cent silicon, from 0.25 to 1.00 per cent m'olybdenum and the balance substantially all iron.

3. A steel for use in oil stills containing carbon in an amount less than 0.10 per cent, mantially all iron.

ganese in an amount less than 0.20 per cent, between 1 and 2 per cent silicon, molybdenum up to 1 per cent and the balance substantially all iron.

4. A steel resistant to stress and scaling at 5 elevated temperatures containing carbon in an amount less than 0.10 per cent, manganese in an amount less than 0.20 per cent, sulphur in an amount less than 0.06 per cent, phosphorus in an amount less than 0.06 per cent, between 1 and 2 per cent silicon, from 0.25 to 1.00 per cent molybdenum and the balance substantially all iron.

5. A steel resistant to stress and to scaling at elevated temperatures containing carbon in an amount less than 0.05 per cent, manganese in an amount less than 0.20 per cent, between 1 and 2 per cent silicon and between 0.25 and 1.00 per cent molybdenum, with the balance substan- 6. A steel resistant to stress and to scaling at elevated temperatures containing carbon in an amount less than 0.10 per cent, manganese in an amount less than 0.20 per cent, between 1 and 2 per cent silicon and between 0.10 and 1.00 per cent molybdenum, with the balance substantially all iron.

WILLIAM R. FLEMING. 

